1. Field of the Invention
Aspects of the present invention relate to a secondary battery, and more particularly, to a secondary battery charging method and device capable of enhancing the stability and extending the life span of a battery.
2. Description of the Related Art
As the use of portable electronic appliances such as mobile phones, notebook computers, camcorders and PDAs grows, secondary batteries used as power sources for these appliances are actively being developed.
Such secondary batteries include nickel-cadmium secondary batteries, lead storage batteries, nickel-metal hydride batteries, lithium ion batteries, and lithium polymer batteries. Generally, there is a demand for secondary batteries that are highly stable, easy to carry and small in size. Moreover, there is a further demand for secondary batteries that can be fully charged in a short time, have a higher capacity and last for more discharge/recharge cycles.
A secondary battery is manufactured as a cell type, and then the cell is coupled to a protective circuit to form a battery pack. The secondary battery is charged from an external power source or discharged to a load through an external terminal included in the battery pack.
The battery pack includes at least one cell. A low-capacity battery having one cell is used in compact and portable electronic appliances such as mobile phones, note book computers, cameras and so on. At the other extreme, a high-capacity battery having several cells is widely used in hybrid cars as a driving power source for the car's engine.
When an external power source is connected to the external terminal and protective circuit, the cell is charged. However, when a load is connected to the external terminal, electrical energy is provided to the load from the charged cell through the protective circuit and external terminal, and thus the cell is discharged.
A constant current-constant voltage (CC-CV) charging method is commonly used as a method of charging a secondary battery. The CC-CV charging method is performed first by charging a secondary battery with constant current, and then by charging the battery with constant voltage when the battery almost reaches its full-charge potential.
To reduce the time necessary for fully charging the battery using only the CC-CV charging method, a charging current has to be set high during the constant current charging phase. However, a high charging current is not preferable because it may degrade the performance of the battery. Therefore, to augment the CC-CV charging method, a pulse charging method has been developed, which may shorten the charging time by repeatedly pulsing between charging and pausing.
Because of repeated charging and discharging, cell degradation occurs, and the degradation degree depends on characteristics of each individual cell. Thus, the degradation degrees of the individual cells become different with time. Therefore, in a secondary battery having more than one cell, the charging/discharging time and amount are different among the cells because of the degradation differences among the individual cells during charging and discharging, and the more degraded cells are charged or discharged more rapidly. However, in a secondary battery having more than one cell, each cell is charged and discharged simultaneously, and the time is controlled according to the state of the first fully-charged or discharged cell. That is, since the most degraded cell is fully charged or discharged first, other less degraded cells are not fully charged or discharged.
Furthermore, the more degradation that occurs in the more degraded cells, the shorter is the charging/discharging time for the entire battery. As a result, the more degraded cells are even further degraded than others, which may result in generation of internal gas, ignition and explosion. Thus, the protective circuit needs to have a circuit for interrupting electrical flow to prevent charging/discharging when the voltage difference among the cells is 300 mV or more in a fully charged or fully discharged battery.
In the conventional art, problems caused by unequal degradation among cells having a voltage difference of 300 mV or more may be solved, but problems occurring when the voltage difference among the cells is less than 300 mV may not be solved. However, although serious safety problems may occur when the voltage difference among the cells is 300 mV or more, there may still be a safety hazard because of unequal degradation among the cells when the voltage difference among the cells is less than 300 mV. Yet, if the voltage difference set point among cells is set too low, the cells that are still usable have to be discarded, which thus brings about an economic loss.